Amazon appeared to just before 4 p.m. (EST), or about an hour into its 36-hour annual promotion. Amazon tweeted a statement just before 5 p.m. (EST) which read in part, "Some customers are having difficulty shopping, and we're working to resolve this issue quickly."

Prime Day, created by Amazon.com Inc. in 2015 to mark its 20th anniversary, has inspired other e-commerce companies to invent their own shopping holidays. Online furniture seller Wayfair introduced Way Day in April, becoming its biggest revenue day ever.

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While Prime Day brings in more revenue for Amazon, too, it also helps boost its Prime memberships. It had more sign-ups during 2017's event than any other day in the company's history, Amazon said at the time, without providing specific numbers.